The television field has been enhanced by two relatively new consumer products: the video tape recorder or video disc player and the personal computer. The video tape recorder or video disc player plays back video tapes or discs for displaying images on the screen of a television receiver or monitor. The personal computer employs the television receiver or monitor as a convenient display for displaying images such as graphics or alphanumerics on the screen thereof. Because of the advent of these two relatively new products, applications have arisen wherein it is desirable to display simultaneously on the television receiver or monitor screen the outputs from a video tape or disc player and a personal computer.
In order to simultaneously display the outputs from these two devices, it is necessary to establish synchronization between them. This is a problem. Video tape recorders, for example, have neither absolute frequency stability nor freedom from flutter, tape slippage and other deviations from an ideal playback medium. In addition, in consumer quality video playback equipment there is no phase or frequency coherence between luminance and chroma signals. During the playback process the latter is reconstructed so that its burst frequency is correct. As a result the central timing source of the personal computer, that is, its system clock, cannot be frequency and phase locked directly to the chroma burst signal (3.58 MHz typically in NTSC systems) in an effort to synchronize the luminance signal of the personal computer with that of the playback unit.
Table I below illustrates the extent to which two different video tape recorders degrade the standard U.S. NTSC television signal, also shown in the table.
TABLE I __________________________________________________________________________ NTSC U-MATIC BETAMAX __________________________________________________________________________ Chroma 3.579545MHz .+-. 0.0003% 3.579545MHz .+-. 0.001% 3.579545MHz .+-. 0.001% sub- Coherent Non-coherent Non-coherent carrier (f.sub.c) Hori- 15,734.264Hz .+-. 0.0003% 15,734Hz .+-. 0.01% 15,730Hz .+-. 0.1% zontal Locked to Chroma Ref. Locked to Speed Servo Locked to Line Freq. Scan- (15,750 monochrome) (15,750 monochrome) (15,750 monochrome) ning Freq. (f.sub.H) Hori- .+-. 0.3 .mu.sec. .+-. 0.250 .mu.sec. .+-. 0.250 .mu.sec. zontal Jitter (.DELTA..tau..sub.H) Verti- 59.94Hz .+-. 0.0003% 59.94Hz .+-. 0.01% 59.94Hz .+-. 0.1% cal Locked to Chroma Ref. Locked to Speed Servo Locked to Line Freq. Scan- (60 Hz monochrome) (60 Hz monochrome) (60 Hz monochrome) ning Freq. __________________________________________________________________________
The lack of coherence of the color subcarrier and its looser frequency tolerance should be noted as well as the fact that the frequency tolerance on the frequency of the horizontal sync signals of the U-MATIC and BETAMAX is roughly three to thirty times worse than that of a standard NTSC signal. Further, it is to be noted that the vertical scanning frequency of the BETAMAX and U-MATIC playback units is similarly degraded over a standard NTSC specification. Relatively low-cost video tape recorders, such as BETAMAX, drive the tape from the capstan of a line-synchronous motor, and therefor, are subject to AC power line frequency variations. The latter in turn are quite variable, with a .+-.0.1% maximum frequency drift not uncommon. As a result both horizontal and vertical frequencies are subject to the same drift.
Because of the non-standard characteristics of video playback devices it is not possible to display images from these simultaneously with images derived from a source of computer controlled video unless some means of synchronization is provided.
Accordingly, it is an object of the present invention to provide a system for achieving synchronization between two video sources.
It is another object of this invention to provide means for synchronizing a source of computer controlled video to another video source.